In general, the present invention relates to the field of communication systems and more particularly, to a communication system that transmits data during repetitive time slots.
In known two-way radio frequency communication networks, a number of transmitter units are used to transmit messages on outbound radio frequency (RF) channels to mobile communication units, for example, two-way pagers, cellular telephones, radios, etc. One or more receivers are associated with each transmitter for receiving messages from mobile communication units on inbound RF channels.
Various paging protocols are used for one-way and two-way paging. For example, the FLEX technology developed by Motorola Inc., the assignee of the present invention, organizes the message into frames of data or a specific sized packet containing bits of data. FLEX and ReFLEX protocols, which specify a one-way and a two-way protocol, respectively, are adopted by many paging service providers worldwide. These protocols are multi-speed, high-performance paging protocols. The protocols are based on a xe2x80x9csynchronousxe2x80x9d time slot protocol, which is designed to increase the battery life of the pagers. Instead of sending out messages at random, all paging data intended for a particular pager is scheduled into a pre-defined time slot for transmission.
There are a total of 128 frames in a FLEX protocol system numbered zero through 127. The frames are transmitted at 32 frames per minute, and thus a full 128-frame cycle lasts four (4) minutes. This arrangement allows a FLEX-based pager to selectively decode one or more frames over each four minute FLEX cycle, so that the pager does not need to waste its battery life decoding data intended for other pagers.
Many selective messaging systems, such as paging systems, are simulcast systems where a message is simultaneously or nearly simultaneously launched from each or all transmitters in a system over outbound channels. These simulcast systems provide excellent coverage in that a mobile communication units is very likely to receive any message intended for the device, regardless of the location of the device or lack of knowledge on the part of the system of its location. The system also includes receivers that receive inbound message traffic transmitted from the mobile communication units.
As a part of the ReFLEX protocol, certain time slots within a frame carry critical fragments of messages that can be lost due to co-channel interference. One such critical fragment is the xe2x80x9cStart Address Unit Responsexe2x80x9d message, which is transmitted by a mobile communication unit as the first packet of a multi-packet scheduled inbound message. The Start Address Unit Response contains data that specifies the length of inbound messages. The mobile communication unit transmits the Start Address Unit Response in response to an outbound Schedule Inbound message command from the system. If the Start Address Unit Response is lost or received in error, all of the following inbound packets, i.e., packets transmitted from the mobile communication units to the system, can not be received correctly, because the system does not know the number of remaining fragments.
Co-channel interference is one of the factors contributing to loss of data in a communication system or device. In order to lower co-channel interference, various techniques have been used to achieve frequency reuse. Under one approach, a number of different RF channels are assigned neighboring coverage areas, or cells. Because of the relatively low power RF transmissions within a particular cell, another cell spaced two or more cells apart may typically reuse the same frequency, in accordance with a frequency reuse pattern. The farther the cells reusing the same frequencies are from each other; the less aggressive the reuse pattern is considered to be. On the other hand, the closer the cells reusing the same frequency are to each other, the more aggressive the reuse pattern is considered. The more aggressive the reuse patterns the higher the achievable data rate and the higher the risk of co-channel interference, and vice versa.
FIGS. 1 and 2 are diagrams of a plurality of cells employing frequency reuse patterns. FIG. 1 illustrates a frequency reuse pattern with a seven-cell cluster. If, for example, the two-way messaging system has twenty-one communication frequencies and each cell utilizes all time slots, then each cell would utilize three unique frequencies. Conversely, another system could utilize, for example, a single frequency and seven non-overlapping time slots, each of the seven time slots being uniquely assigned to each of the seven cells. Other combinations are possible, so long as each cell communicates on either a different frequency or a different time slot from that used by any other cell. As the seven-cell clusters are repeated throughout the communication system, each cell is susceptible to several sources of communication interference.
As described before, these sources include co-frequency (or co-channel) interference, and adjacent channel interference. Multiple cells outside of the seven-cell cluster utilizing the same communication frequency cause co-channel interference. The communication system is designed to place cells utilizing the same communication frequency as far apart as allowed by the frequency reuse pattern. Adjacent channel interference occurs from communication frequencies that are adjacent to each other in the frequency spectrum. The final form of interference always present in the two-way messaging system is noise inherent in transmitters, receivers, and mobile communication units.
Changing the frequency reuse pattern to increase the frequency reuse distance can reduce co-channel and adjacent channel interference. For example, FIG. 2 illustrates a frequency reuse pattern including twelve-cell clusters. In a two-way messaging system utilizing twenty-one communication frequencies, each cell could simultaneously use an average of 1.75 frequencies. The change in the frequency reuse pattern reduces the system capacity by approximately 42 percent. However, the distance between cells simultaneously utilizing the same communication frequencies has been increased, thereby reducing co-channel interference. In addition, since there are fewer frequencies present within each cell, adjacent channel interference is reduced. By increasing the frequency reuse distance, the coverage range and reliability of the transmitters and receivers in each cell is increased.
Under an inbound frequency reuse approach, a group of receivers receive messages from a transmitter within a coverage area, where the transmitter can be scheduled to transmit data during times that may or may not overlap with each other. If an aggressive reuse is used, the chances are higher for reception of simultaneously transmitted messages at a larger number of receivers that cover a reception footprint. More aggressive reuse, however, may result in co-channel interference that reduces the chances of decoding data, including critical data, correctly.
Thus, if an aggressive reuse is used, i.e., when multiple mobile communicators transmit on the same frequency at the same time, enough co-channel interference can be caused that results in loss of data. This is a known risk of reusing the frequency for simultaneous transmissions. In the case of scheduling a single or a small number of packets, this is worth the risk, mainly because the requirement for retransmission and recovery of a small amount of erroneously received or lost data is manageable. But in long data transmissions, for example 115 packets, if a packet that contains critical data is lost, all of the 115 packets become unrecoverable. The requirement for retransmission or recovery of such a large amount of erroneously received or lost data can degrade the overall system throughput and performance.
Therefore, there exists a need to improve system performance by reducing data loss due to frequency reuse on a time slot by time slot basis.